Flow cytometry is a technology that can simultaneously measure and then analyze physical characteristics of single particles, usually cells, as they flow in a fluid stream through a beam of light. Many biochemical procedures require isolating cells of a uniform type from a tissue containing a mixture of cell types.
For example, cell separation is often used to analyze the DNA content of individual cells. In a typical cell-separation technique, DNA or antibodies coupled to a fluorescent dye are used to label specific cells. The labeled cells can then be separated from the unlabeled cells in a flow cytometer, also known as a fluorescence-activated cell sorter. A typical flow cytometer lines particles up in single file in a fine stream. The individual cells traveling single file pass through a laser beam and the fluorescence of each cell is measured. Selected cells can then be separated from the fluid stream using a vibrating nozzle to form droplets containing single cells. This technique can sort many thousands of cells per second.
The cytometer fluid system must transport the particles in a fluid stream to the laser beam for interrogation. For optimal illumination, the particles should be positioned in the center of the laser beam and only one cell or particle should move through the laser beam at a given moment. However, as shown in FIG. 1A, under normal pressure-driven laminar flow in a microfluidic channel, the fluid velocity profile is governed by a parabolic relation: fluid velocities near channel walls go to zero, while fluid velocities near the center of the channel are maximized. Particles carried along fluid streamlines will thus observe the same fluid velocity profile, with particles near walls moving slowly and particles in the center moving rapidly. As shown in FIG. 1B, this presents problems for optical detection schemes that rely on uniform particle velocities. With integration detectors, such as a photodetector, the detector will be unable to differentiate between a large number of fast moving particles and a low number of slow moving particles since both cases will produce similar optical signals. In addition, particles at different planes in the channel will be either in or out of focus of the optical detection system, also giving variability in optical detection signals.
To solve these problems, most conventional flow cytometers use hydrodynamic focusing, wherein the sample is injected into a stream of shealth fluid within the flow channel. The sample core remains separate but coaxial within the shealth fluid, enabling the laser beam to interact with the particles that are focused to a thin-core fluid streamline. However, hydrodynamic focusing requires that additional flow streams be added to the system, complicating fabrication, fluid routing, and waste disposal. Therefore, a need remains for a simplified focusing device to confine microparticles within fluid streamlines inside a microfluidic channel.